Myeloperoxidase (MPO) is a heme-containing enzyme found predominantly in polymorphonuclear leukocytes (PMNs). MPO is one member of a diverse protein family of mammalian peroxidases that also includes eosinophil peroxidase, thyroid peroxidase, salivary peroxidase, lactoperoxidase, prostaglandin H synthase, and others. The mature enzyme is a dimer of identical halves. Each half molecule contains a covalently bound heme that exhibits unusual spectral properties responsible for the characteristic green colour of MPO. Cleavage of the disulphide bridge linking the two halves of MPO yields the hemi-enzyme that exhibits spectral and catalytic properties indistinguishable from those of the intact enzyme. The enzyme uses hydrogen peroxide to oxidize chloride to hypochlorous acid. Other halides and pseudohalides (like thiocyanate) are also physiological substrates to MPO.
PMNs are of particular importance for combating infections. These cells contain MPO, with well-documented microbicidal action. PMNs act non-specifically by phagocytosis to engulf microorganisms, incorporate them into vacuoles, termed phagosomes, which fuse with granules containing myeloperoxidase to form phagolysosomes. In phagolysosomes the enzymatic activity of the myeloperoxidase leads to the formation of hypochlorous acid, a potent bactericidal compound. Hypochlorous acid is oxidizing in itself, and reacts most avidly with thiols and thioethers, but also converts amines into chloramines, and chlorinates aromatic amino acids. Macrophages are large phagocytic cells which, like PMNs, are capable of phagocytosing microorganisms. Macrophages can generate hydrogen peroxide and upon activation also produce myeloperoxidase. MPO and hydrogen peroxide can also be released to the outside of the cells where the reaction with chloride can induce damage to adjacent tissue.
Linkage of myeloperoxidase activity to disease has been implicated in neurological diseases with a neuroinflammatory response including multiple sclerosis, Alzheimer's disease, Parkinson's disease and stroke as well as other inflammatory diseases or conditions like asthma, chronic obstructive pulmonary disease, cystic fibrosis, atherosclerosis, ischemic heart disease, heart failure, inflammatory bowel disease, renal glomerular damage and rheumatoid arthritis. Lung cancer has also been suggested to be associated with high MPO levels.
Multiple Sclerosis (MS)
MPO positive cells are immensely present in the circulation and in tissue undergoing inflammation. More specifically MPO containing macrophages and microglia has been documented in the CNS during disease; multiple sclerosis (Nagra R M, et al. Journal of Neuroimmunology 1997; 78(1-2):97-107), Parkinson's disease (Choi D-K. et al. J. Neurosci. 2005; 25(28):6594-600) and Alzheimer's disease (Green P S. et al. Journal of Neurochemistry. 2004; 90(3):724-33). It is supposed that some aspects of a chronic ongoing inflammation result in an overwhelming destruction where agents from MPO reactions have an important role.
The enzyme is released both extracellularly as well as into phagolysosomes in the neutrophils (Hampton M B, Kettle A J, Winterbourn C C. Blood 1998; 92(9): 3007-17). A prerequisite for the MPO activity is the presence of hydrogen peroxide, generated by NADPH oxidase and a subsequent superoxide dismutation. The oxidized enzyme is capable to use a plethora of different substrates of which chloride is most recognized. From this reaction the strong non-radical oxidant—hypochlorous acid (HOCl)— is formed. HOCl oxidizes sulphur containing amino acids like cysteine and methionine very efficiently (Peskin A V, Winterbourn C C. Free Radical Biology and Medicine 2001; 30(5): 572-9). It also forms chloramines with amino groups, both in proteins and other biomolecules (Peskin A V. et al. Free Radical Biology and Medicine 2004; 37(10):1622-30). It chlorinates phenols (like tyrosine) (Hazen S L. et al. Mass Free Radical Biology and Medicine 1997; 23(6): 909-16) and unsaturated bonds in lipids (Albert C J. et al. J. Biol. Chem. 2001; 276(26): 23733-41), oxidizes iron centers (Rosen H, Klebanoff S J. Journal of Biological Chemistry 1982; 257(22): 13731-354) and crosslinks proteins (Fu X, Mueller D M, Heinecke J W. Biochemistry 2002; 41(4): 1293-301).
Proteolytic cascades participate both in cell infiltration through the BBB as well as the destruction of BBB, myelin and nerve cells (Cuzner M L, Opdenakker G. Journal of Neuroimmunology 1999; 94(1-2): 1-14; Yong V W. et al. Nature Reviews Neuroscience 2001; 2(7):5 02-11). Activation of matrix metalloproteinases (MMPs) can be accomplished through the action of upstream proteases in a cascade as well as through oxidation of a disulfide bridge Fu X. et al. J. Biol. Chem. 2001; 276(44): 41279-87; Gu Z. et al. Science 2002; 297(5584): 1186-90). This oxidation can be either a nitrosylation or HOCl-mediated oxidation. Both reactions can be a consequence of MPO activity. Several reports have suggested a role for MMP's in general and MMP-9 in particular as influencing cell infiltration as well as tissue damage (BBB breakdown and demyelination), both in MS and EAE (for review see Yong V W. et al, supra). The importance of these is specific kinds of mechanisms in MS comes from studies where increased activity and presence of proteases have been identified in MS brain tissue and CSF. Supportive data has also been generated by doing EAE studies with mice deficient in some of the proteases implicated to participate in the MS pathology, or by using pharmacological approaches.
The demyelination is supposed to be dependent on the cytotoxic T-cells and toxic products generated by activated phagocytes (Lassmann H. J Neurol Neurosurg Psychiatry 2003; 74(6): 695-7). The axonal loss is thus influenced by proteases and reactive oxygen and nitrogen intermediates. When MPO is present it will obviously have the capability of both activating proteases (directly as well as through disinhibition by influencing protease inhibitors) and generating reactive species.
Chronic Obstructive Pulmonary Disease (COPD)
Chronic obstructive pulmonary disease (COPD) is a disease state characterised by airflow limitation that is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles or gases. COPD is a major public health problem. It is the fourth leading cause of chronic morbidity and mortality in the United States and is projected to rank fifth in 2020 as a worldwide burden of disease. In the UK the prevalence of COPD is 1.7% in men and 1.4% in women. COPD spans a range of severity from mild to very severe, with the cost of treatment rising rapidly as the severity increases.
Levels of MPO in sputum and BAL are much greater in COPD patients than normal, non-smoking controls (Keatings V. M., Barnes P. J. Am. J Respir Crit. Care Med 1997; 155:449-453; Pesci, A. et al. Eur Respir J 1998; 12:380-386). MPO levels are further elevated during exacerbations of the disease (Fiorini G. et al. Biomedicine & Pharmacotherapy 2000; 54:274-278; Crooks S. W. et al. European Respiratory Journal. 15(2): 274-80, 2000). The role of MPO is likely to be more important in exacerbations of COPD (Sharon S. D. et al. Am J Respir Crit. Care Med. 2001; 163: 349-355).
In addition to the destructive capacity of MPO there is a strong clinical link with vascular disease (Baldus S. et al. Circulation 2003; 108: 1440-5). Dysfunctional MPO polymorphisms are associated with a reduced risk of mortality from coronary artery disease (Nikpoor B. et al. Am Heart J 2001; 142: 336), and patients with high serum levels of MPO have increased risk of acute coronary syndrome. The effects of MPO on vascular disease may extend to COPD, since there is strong evidence that the pulmonary vasculature is one of the earliest sites of involvement in the smokers' lung. Striking changes in the intima of the pulmonary arteries have been described which show a dose relationship with smoking (Hale K. A., Niewoehner D. E., Cosio M. G. Am Rev Resp Dis 1980; 122: 273-8). The physiological function of MPO is associated with innate host defense. This role, however, is not critical as most cases of MPO deficient patients have relatively benign symptoms (Parry M. F. et al. Ann Int Med. 1981; 95: 293-301, Yang, K. D., Hill, H. R. Pediatr Infect Dis J. 2001; 20: 889-900). In summary, there is considerable evidence that elevated MPO levels in COPD may contribute to the disease via several mechanisms. A selective inhibitor of MPO would therefore be expected to alleviate both the acute and chronic inflammatory aspects of COPD and may reduce the development of emphysema.
Atherosclerosis
An MPO inhibitor should reduce the atherosclerotic burden and/or the vulnerability of existing atherosclerotic lesions and thereby decrease the risk of acute myocardial infarction, unstable angina or stroke, and reduce ischemia/reperfusion injury during acute coronary syndrome and ischemic cerebrovascular events. Several lines of data support a role for MPO in atherosclerosis. MPO is expressed in the shoulder regions and necrotic core of human atherosclerotic lesions and active enzyme has been isolated from autopsy specimens of human lesions (Daugherty, A. et al. (1994) J Clin Invest 94(1): 437-44). In eroded and ruptured human lesions, as compared to fatty streaks, an increased number of MPO expressing macrophages have been demonstrated, suggesting a particular role for MPO in acute coronary syndromes (Sugiyama, S. et al. (2001) Am J Pathol 158(3): 879-91). Patients with established coronary artery disease have higher plasma and leukocyte MPO levels than healthy controls (Zhang, R. et al. (2001) Jama 286(17): 2136-42). Moreover, in two large prospective studies plasma levels of MPO predicted the risk of future coronary events or revascularisation (Baldus, S. et al. (2003) Circulation 108(12): 1440-5; Brennan, M. et al. (2003) N Engl J Med 349(17): 1595-604). Total MPO deficiency in humans has a prevalece prevalence of 1 in 2000-4000 individuals. These individuals appear principally healthy but a few cases of severe Candida infection have been reported. Interestingly, MPO deficient humans are less affected by cardiovascular disease than controls with normal MPO levels (Kutter, D. et al. (2000) Acta Haematol 104(1)). A polymorphism in the MPO promoter affects expression leading to high and low MPO expressing individuals. In three different studies the high expression genotype has been associated with an increased risk of cardiovascular disease (Nikpoor, B. et al. (2001) Am Heart J 142(2): 336-9; Makela, R., P. J. Karhunen, et al. (2003) Lab Invest 83(7): 919-25; Asselbergs, F. W., et al. (2004) Am J Med 116(6): 429-30). Data accumulated during the last ten years indicate that the proatherogenic actions of MPO include oxidation of lipoproteins, induction of endothelial dysfunction via consuming nitric oxide and destabilisation of atherosclerotic lesions by activation of proteases (Nicholls, S. J. and S. L. Hazen (2005) Arterioscler Thromb Vasc Biol 25(6): 1102-11). Recently, several studies have focused on nitro- and chlorotyrosine modifications of LDL and HDL lipoproteins. Since chlorotyrosine modifications in vivo only can be generated by hypochlorus acid produced by MPO these modifications are regarded as specific markers of MPO activity (Hazen, S. L. and J. W. Heinecke (1997) J Clin Invest 99(9): 2075-81). LDL particles exposed to MPO in vitro become aggregated, leading to facilitated uptake via macrophage scavenger receptors and foam cell formation (Hazell, L. J. and R. Stocker (1993) Biochem J 290 (Pt 1): 165-72). Chlorotyrosine modification of apoA1, the main apolipoprotein of HDL cholesterol, results in impaired cholesterol acceptor function (Bergt, C., S. et al. (2004) Proc Natl Acad Sci USA; Zheng, L. et al. (2004) J Clin Invest 114(4): 529-41). Systematic studies of these mechanisms have shown that MPO binds to and travels with apoA1 in plasma. Moreover, MPO specifically targets those tyrosine residues of apoA1 that physically interact with the macrophage ABCA1 cassette transporter during cholesterol efflux from the macrophage (Bergt, C. et al. (2004) J Biol Chem 279(9): 7856-66; Shao, B. et al. (2005) J Biol Chem 280(7): 5983-93; Zheng et al. (2005) J Biol Chem 280(1): 38-47). Thus, MPO seems to have a dual aggravating role in atherosclerotic lesions, i.e. increasing lipid accumulation via aggregation of LDL particles and decreasing the reverse cholesterol transport via attack on the HDL protein apoA1.
The present invention discloses novel thioxanthines that surprisingly display useful properties as inhibitors of the enzyme MPO. Furthermore, the novel compounds of the present invention display either one or more than one of the following: (i) improved selectivity towards TPO; (ii) unexpectedly high inhibitory activity towards MPO; (iii) improved brain permeability; (iv) improved solubility and/or (v) improved half-life; when compared to known thioxanthines. Such thioxanthines are disclosed in e.g. WO 03/089430 and WO 05/037835.